def read_big_file(f,splitStr):
    buf=""
    while True:
        while splitStr in buf:
            pos= buf.index(splitStr)
            yield buf[:pos]
            buf=buf[pos+len(splitStr):]
        chunk=f.read(2)

        if not chunk:
            #已经读到了最后
            yield buf
            break
        buf+=chunk



if __name__=="__main__":
    with open('bigfile.txt') as f:
        for chunk in read_big_file(f,","):
            print(chunk)